A light guide can be used to transport light for illumination. For example, a light guide can be used as the backlight, e.g., of a liquid crystal display (LCD) in many applications, including computer monitors, television, instrument panels, aircraft or marine craft cockpit displays, signage, and consumer devices such as video players, gaming devices, clocks, watches, calculators, still picture cameras, video cameras, mobile/cell phones and other telephones. Moreover, scanning backlights are slim backlights which emit collimated rays whose direction of collimation can be scanned. A light guide can also be used to provide a projection display, such as in head-mounted displays which are used in various applications, including military, aviation, medicine, video gaming, entertainment, sports, and so forth. See-through head-mounted displays allow the user to observe the physical world around him or her, while optical elements add light into the user's visual path, to provide an augmented reality image. However, it is difficult to provide uniform illumination for such light guides.